Wow, there are people who STILL deny the solid and obvious fact that is evolution? Well, I guess the condition of being sentient kind of subverts the "survival of the fittest" thing. This one dude, he married this lady, she was like 400 lbs...ANYWAY, the resulting child-thing, what with the combined genes of its parents, will a be new specimen added to humanity. A new specimen is different from other specimens, thus, genes are mixed & matched when this specimen procreates, leading to even more change within the human gene pool. Change adds up over time, thus, evolution. Even if it results in the downfall of mankind.
Now, let's go to wildlife. Wildlife does the evolution the right way. Why? Because if you're a morbidly obese sheep, you can't outrun that bear who's keen on eating you, and therefore you don't get to spread your fat ass genes. Sucks to be you. Now, on the other hand, if you're a sheep born with a slight genetic mutation that makes you physically stronger than most sheep, you're going to have a much better chance of outrunning that bear and living to spread your super genes. Very gradually, as more and more sheep with super genes and less and less sheep with fat ass genes mate, the sheep gene pool will be significantly swayed. As a result, a species of sheep who is better adapted to survive bear attacks will evolve. At the same time, bears will have evolved to be better equipped to take down sheep, since bears who could catch sheep were able to mate while bears who couldn't feed themselves didn't get to score. The process continues forever, until something catastrophic, like a meteor or urban development, wipes out a species faster than it can procreate.
Waterbears can survive almost ANYTHING because only the toughest waterbears were able to mate frequently enough to have a profound effect on the genepool. This is a prime example of evolution.